Our first stay at Vistana (Villages, not Resort) was through Air Canada Vacations, but there would be no issues with booking through Costco. They are a TICO agency, so your vacation has all the protections of any other
travel agent, and Costco books a TON of travel. Yes, Costco rates are often outstanding.
The 60 day FP is extended to the Disney Springs area hotels only. This does not include SVR - it's close to Disney Springs, but not on Hotel Plaza. EMH is not available at any offsite resorts that I am aware of. Still, I generally avoid the EMH park each day like the plague. EMH attracts larger crowds to the park that is offering it that day, and the smart touring tip is to go to any other park. Unless you are travelling at Christmas, it is pretty easy to get most FP+ at 30 days; maybe not the prime times, but if you play it well, you can still have a power touring experience. We are March break travellers and got FP+ for everything we wanted but FoP this year, which had only been open for weeks.
As a good neighbor hotel, SVR does offer free shuttles to hotel stays (like the Costco booking), but your own vehicle is still highly recommended.